Basically the title explains it all, however, I don't mean like someone who has written books on the Civil War and Reconstruction. While each subject has distinct historiographical developments, they are (in the grand scheme of things) pretty similar.
I'm interested in someone who has perhaps written on subjects like the Civil War and Medieval France; academic subjects that have nothing in common with each other and requires a mastery of two very different fields of inquiry.
There are a lot of Historians who publish on different historical fields, but it depends on what kind of book you mean. I don't recall someone working extensively on one small historical phenomenon (in a doctoral dissertation, for example) and then doing the same for another field, perhaps centuries apart. But there are a lot of Historians, mostly among the older, well-known ones, who try to prove their narrative through looking at a much wider scale of time.
Best example that came to my mind is the german historian Heinrich August Winkler, who in 2000 published his book "Germany: The Long Road West" which portrays german history from about 1789 to 1990. In 2009 he published his "History of the West" which even starts out in ancient history. Keep in mind though that those books are works of synthesis and do not require extensive work on primary sources like dissertations would.
In Germany, Historians who want to become professors are expected to work on unrelated subjects in their Dissertation (PHD thesis) and Habilitationsschrift (book required to become a professor). But this in most times means different aspects of the same field.